This invention relates, in general, to growing single crystal ingots of a semiconductor material. More particularly, the present invention pertains to growing large diameter crystals.
The preferred method of producing single crystal ingots of semiconductor materials is by a technique called the Czochralski (CZ) method. Using the CZ method to grow a single crystal silicon ingot includes forming a melt of polycrystalline silicon within a crucible heated by resistance heating or high frequency heating. A single crystal seed is attached to a seed holder that is connected to an end of a pull shaft or cable and dipped into the melt of polycrystalline silicon. Subsequently, the crystal is pulled from the melt while rotating at a specified speed. The diameter of the crystal can be controlled by changing the temperature of the melt and/or by changing the rate at which the crystal is pulled from the melt. Generally, the temperature of the melt is altered by changing the power provided to the heater.
A well known problem in growing a single crystal ingot is the formation of dislocations in the single crystal ingot. This problem was overcome by beginning the crystal pull relatively quickly to form a thin neck. It is essential to the process to reduce the diameter of the seed crystal to approximately 3 to 6 millimeters (mm) to achieve a zero dislocation crystal. This seeding method is typically referred to as the Dash technique (W. C. Dash, J. Appl. Physics, Vol. 30, pp. 459-474, 1959). Once a zero dislocation crystal has been achieved, the pull speed is reduced, resulting in the growth of a larger diameter body from the thin neck. This is a conventional method for growing single crystal ingots having diameters less than 200 mm, and typically 125 to 150 mm in diameter.
Currently, the semiconductor industry is interested in larger diameter ingots. The problem is that when a large diameter single crystal ingot is being grown, the thin neck is subjected to more stress than it can structurally handle. Two stresses affect the thin neck, tensile stress from the growing weight of the ingot, and torsion stress from the rotational viscosity drag force of the solid-liquid interface. Both stresses increase with an increase in the diameter of the single crystal. If these combined stresses become larger than the yield strength of the thin neck, the thin neck may break, or more commonly, may generate dislocations in the single crystal. The yield strength of silicon single crystals reduces with increasing temperature. Therefore, when growing the large diameter crystal, the combination of stresses may exceed the yield strength at the end of the thin neck when the temperature is high.
As is readily apparent, methods for growing and structures are needed for large diameter semiconductor ingots that remedy the foregoing and other deficiencies inherent in the prior art.